Carbon is an essential element and intimately involved in nearly all biochemical pathways. Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) can be used to noninvasively explore structure and properties of 13C compounds. Compared to other nuclei, 13C has large chemical shift dispersion; for example, the 13C chemical shift range is about 30-fold greater than that of protons. When 13C enriched compounds are used, the NMR spectrum is simpler to interpret since only the NMR signals originating from the administered compounds and corresponding metabolites are observed.
However, the analytical potential of 13C NMR has not been exploited to its full capacity because of poor sensitivity. The low sensitivity of 13C NMR/MRI originates from the inherently low natural abundance (1.1%) and small gyromagnetic ratio, γ13C=γ1H/4. Thus, 13C nuclear spin polarization (fraction of nuclear spins contributing to MR signal), is only 4×10−6 at 4.7 T. For comparison, the polarization produced by optical methods is close to unity.
The low thermal 13C spin polarization can be enhanced by polarization transfer to carbon from more abundant protons. A number of approaches have been developed to enhance the thermal 13C polarization by as much as a factor of four. However, the transfer of thermal polarization leads to only a minor improvement and leaves the sensitivity inadequate for the majority of in vivo applications.
An alternative approach to enhance 13C NMR signal is to create non-thermal polarization, often referred as “hyperpolarization”. Hyperpolarization methods include optical pumping of noble gases, PASADENA (Parahydeogen and Synthesis Allow Dramatically Enhanced Nuclear Alignment),2,3 and DNP (Dynamic Nuclear Polarization)4,5. These methods can potentially create a non-thermal polarization close to unity, thereby increasing 13C NMR signals by factor 104-105.
The PASADENA effect relies on the molecular addition of para-hydrogen to a double bond in a substrate molecule via catalytic hydrogenation. The spin polarization of adjacent protons can be transferred to 13C by various methods6,7. DNP is based on polarizing nuclear spins in the solid state at cryogenic temperatures and utilizes the transfer of spin polarization from electrons to the nuclear spins. This mechanism requires the presence of unpaired electrons, which are added to the sample as organic free radicals. Both DNP and PASADENA have been shown to be particularly useful for enhancing the signals of some carbon compounds8.
Despite recent successes of PASADENA7-11 and DNP,12-15 these methods have significant limitations. For example, the PASADENA method is limited to unsaturated molecular precursors with the C═C bond that allow molecular addition of para-hydrogen to produce hyperpolarized molecular agents. DNP requires high field magnets, expensive cryogenic equipment, and several tens of minutes to produce hyperpolarized samples. Moreover, DNP and PASADENA utilize toxic chemicals (free radical or catalyst respectively), which require filtration for use in vivo.
The nuclear spin polarization of the noble gases 3He and 129Xe can be enhanced by several orders of magnitude (up to a factor of 105) by optical pumping methods. Hyperpolarized noble gases are produced by transferring the angular momentum from a beam of circularly polarized laser light to the atom's nuclear spins via an intermediate alkali metal vapor step16-20. Removal of the alkali metal from the hyperpolarized gas is easily accomplished by a simple freeze out, which is a part of polarization procedure. Importantly, this can be accomplished with virtually no loss of polarization. In the case of PASADENA and DNP filtering out toxic substances requires an extra step upon which some polarization losses might occur. Hyperpolarized noble gases are easily administered via inhalation and therefore, have crucial advantage for in vivo applications, most importantly for human studies. Hyperpolarized noble gas MRI has allowed visualization of the sinuses21, perfusion in the brain22,23 and the lung24-32, with main research effort being devoted to the imaging of lung structure and function.
Hyperpolarized 129Xe can also be used to enhance 13C polarization by spin polarization-induced nuclear Overhauser effect (SPINOE)33,34 or low-field thermal mixing (TM).4,5,45,36 SPINOE is the result of cross-relaxation between hyperpolarized xenon and other nuclear species. The low-field thermal mixing is performed in a frozen mixture of the hyperpolarized xenon and 13C-containing compound. TM is driven by the dipolar interactions between the nuclear spins and requires a uniform distribution of 13C spins in a solid 129Xe matrix. When the applied magnetic field (referred to as a “mixing field”) is comparable to the local dipolar fields, nuclear spins equilibrate their polarizations.
Although several attempts to transfer polarization from hyperpolarized 129Xe to 13C spins in the solid state by TM have been implemented,35,36 the measured transfer efficiency has always been substantially lower than that predicted by theory. A major requirement of the transfer techniques is the formation of a homogeneous phase by 129Xe and 13C species. As a result, prior methods have been limited to compounds that dissolve well in liquid xenon. Thus, the reported TM studies have all used carbon disulphide (13CS2) due to its good solubility in liquid xenon. The majority of biomolecules, however, have poor xenon solubility, which poses a severe limitation for biologically relevant applications. Hence, there is a need for the development of polarization transfer methods that would be effective for use with biologically relevant 13C containing compounds, particularly those that may be insoluble in xenon liquid.